The printing industry uses a variety of processes, including flexographic, letter press, dry-offset, intaglio, gravure, and the like, to print multiple ink images (often millions of copies) from a single, cylindrical printing plate onto a printing medium. In the printing operation, the somewhat flexible cylindrical printing plate is mounted onto the outer surface of a printing cylinder. The cylindrical printing plate can be mounted onto and removed from the printing cylinder so that a single printing cylinder can perform multiple jobs.
One type of cylindrical printing plate is formed of helically wound polyester. Wound cylindrical printing plates are expanded slightly by air pressure radially applied through holes in the external surface of the printing cylinder to axially mount the plates on, and to remove them from, the printing cylinder. Such "wound" cylindrical printing plates encounter, however, a number of problems. They cannot be expanded effectively unless production facilities purchase auxiliary sources of compressed air. Leakage problems are inherent in the design of wound cylindrical printing plates. Finally, wound cylindrical printing plates provide minimal structural integrity and exhibit minimal strength and durability.
Another type of cylindrical printing plate is made of metal, typically nickel, and may have a photopolymer coated on the metal surface. Air-mount cylinders also may be used to mount and remove such metal cylindrical printing plates. As in the case of wound cylindrical printing plates, metallic cylindrical printing plates are readily expandable but they must have thin walls (typically about 0.005 inches) to expand sufficiently for proper mounting on and removal from the printing cylinder. The required thin wall thicknesses create problems. For example, metallic cylindrical printing plates lack durability and are readily damaged. They easily form kinks in their outer surface when stored without the support of a printing cylinder. The metallic cylindrical printing plates also are limited in the wall thicknesses which they can achieve by electroplating and the metal must often be machined to obtain a variety of wall thicknesses. Finally, metal surfaces tend to add expense.
Dimensional stability is a problem in printing applications which require that the outer surface of the cylindrical printing plate have a truly round shape. In some printing applications, the true cylindrical shape must be within 0.001-0.005 inches tolerance level. The outer printing surface in such applications must accurately conform to a uniformly constant, cylindrical shape to imprint accurately a print image onto a printing medium. Whether wound or metal, conventional cylindrical printing plates often fail to meet these requisite tolerance levels.
A third type of cylindrical printing plate, a photopolymer cylindrical printing plate, is widely used in the printing industry because it has many advantages over wound and metallic cylindrical printing plates. Specifically, photopolymer cylindrical printing plates of a desired wall thickness can be both air mounted and used with conventional hydraulic or mechanical press cylinders. The photopolymer materials provide a fixed internal diameter and a variable outside diameter because a predetermined amount of material can be coated on the cylindrical sleeve during the manufacturing process. Dimensional stability is excellent and leakage problems are avoided. Photopolymer materials also provide strong and durable surfaces having structural integrity.
In medical, industrial, and other unique graphic arts fields, the printing images are continuous; the image areas of the printed matter neither has nor contains a gap. If a sheet or wrapped-butted photopolymer cylindrical printing plate is used, however, a discontinuity or gap will be produced in the image area of the printed matter. Consequently, photopolymer cylindrical printing plates are preferably unitary structures having substantially seamless inner and outer cylindrically shaped wall surfaces.
Even seamless photopolymer cylindrical printing plates suffer, however, from certain disadvantages, including softness and susceptibility to attack by organic solvents Such disadvantages may make conventional photopolymer cylindrical printing plates unsuited for printing applications which involve mechanical stress, surface wear, or inks having solvents or abrasive components. In intaglio or gravure printing, for example, doctor blades wear against the print surface, the inks have strong solvents, and the printing process requires high speeds and long runs.
Therefore, to overcome the shortcomings of existing cylindrical printing plates, a new, seamless, trilaminate, photopolymer cylindrical printing plate is provided which is capable of large printing runs at high speeds with minimal degradation-.even when inks having strong solvents are used. A method of manufacturing such an improved printing plate is also provided.